How this calculator works
This calculator uses the U.S. Navy circumference method, developed by Hodgdon and Beckett at the Naval Health Research Center in 1984 and still used in the Navy's body composition assessment. It predicts body density from the log10 of a few tape-measure circumferences, then converts density to a body fat percentage with the Siri (1961) equation.
Enter your neck and waist (plus hip if you selected female), and your height. Add your body weight to also see fat mass and lean mass in pounds or kilograms, and your category on the American Council on Exercise (ACE) body fat scale. These are estimates, not medical advice — the method has a published accuracy of roughly plus or minus 3-4 percentage points against underwater weighing, and a single tape measurement can shift the result more than that if taken loosely or in the wrong spot.
The U.S. Navy method
Men: body density D = 1.0324 − 0.19077 × log10(waist − neck) + 0.15456 × log10(height)Women: body density D = 1.29579 − 0.35004 × log10(waist + hip − neck) + 0.22100 × log10(height)Body fat % = 495 ÷ D − 450 (Siri equation, 1961)All circumferences and height are in inches. Waist is measured at the navel for men and at the narrowest point for women; hip is measured at the widest point and only enters the women's formula.
Worked example: man, height 70 in, neck 15 in, waist 34 in
- Waist minus neck: 34 − 15 = 19 inches.
- D = 1.0324 − 0.19077 × log10(19) + 0.15456 × log10(70) ≈ 1.0736.
- Body fat % = 495 ÷ 1.0736 − 450 ≈ 11.1%.
- At 180 lb body weight, that is about 20.0 lb of fat mass and 160.0 lb of lean mass.
Frequently asked questions
Why does the formula need neck and hip measurements?
The Navy method infers body fat from the shape of fat distribution rather than a single skinfold. Waist relative to neck (and hip, for women) correlates with total body fat closely enough for a quick estimate, without calipers or a scan.
How accurate is this compared to DEXA or hydrostatic weighing?
The original validation reported roughly plus or minus 3-4 percentage points against underwater weighing, which is decent for a tape measure but well short of lab methods. Use it to track trends over time rather than as a precise single number.
What are the ACE categories based on?
The American Council on Exercise publishes commonly cited body fat ranges: essential fat, athletes, fitness, acceptable and obese, with different thresholds for men and women because women require more essential fat for hormonal function.
Why do I need a weight entry if the percentage doesn't use it?
Body fat percentage comes only from your circumferences and height. Weight is used afterward to split that percentage into fat mass and lean mass in real pounds or kilograms.
Is this medical advice?
No. This tool gives an estimate for general fitness tracking. For clinical decisions about body composition or weight, talk to a doctor or registered dietitian.